WO2011006638A1

WO2011006638A1 - Process for the synthesis of paliperidone

Info

Publication number: WO2011006638A1
Application number: PCT/EP2010/004258
Authority: WO
Inventors: Milos Ruzic; Anica Pecavar; Darja Prudic; Igor Plaper; Andrej Klobcar
Original assignee: Krka, D.D., Novo Mesto
Priority date: 2009-07-13
Filing date: 2010-07-13
Publication date: 2011-01-20
Also published as: EP2275423A1; EP2275423B1; EP2454256A1; SI2454256T1; SI2275423T1; EP2275423B8; EP2275423B9; ATE544766T1; EP2454256B1

Abstract

The invention describes an improved process for the synthesis and purification of paliperidone.

Description

PROCESS FOR THE SYNTHESIS OF PALIPERIDONE

FIELD OF THE INVENTION

BACKGROUND OF THE INVENTION

Paliperidone, 3-[2-[4-(6-fluoro-l,2-benzisoxazol-3-yl)-l-piperidinyl]ethyl]-6,7,8,9- tetrahydro-9-hydroxy-2-methyl-4H-pyrido[l,2-a]pyrimidin-4-one, is a 5-HT antagonist belonging to the chemical class of benzisoxazole derivatives and a racemic mixture has the following structural formula (I):

(I)

Paliperidone is a primary active metabolite of risperidone. Marketed under the name Invega^® paliperidone is a psychotropic agent approved for the treatment of schizophrenia.

A process for the synthesis of paliperidone, is described e.g. in EP 0 368 388 where 6- fluoro-3-(4-pyperidinyl)-l,2-benzisoxazole or its acid addition salts of formula (II)

(H) is reacted with 3-(2-chloroethyl)-9-hydroxy-2-methyl-6,7,8,9-tetrahydro-4H-pyrido[l,2- a]pyrimidin-4-one of formula (III):

(III).

The process is performed in the presence of an organic base and involves isolations using column chromatography and re-crystallizations in suitable solvents. The most persistant impurity to be removed from the active ingredient is 3-[2-[4-(6-fluoro-l,2-benzoisoxazol- 3-yl) 1 -piperidinyl]ethyl]-2-rnethyl-7,8-dihydro-4H-pyrido[ 1 ,2-a]pyrimidin-4,9(6H)-dione (IV) of formula

(IV),

which is very difficult to eliminate due to its chemical similarity with paliperidone, its keto-enol tautomery and its ability to form hydrates.

A process for the synthesis of CMHTP is described in EP 0 368 388, WO 2008/024415, WO 2008/140646, WO 2009/010988, WO 2009/045489 and WO 2009/047499.

WO 2008/021345 describes the synthesis of paliperidone from 3-(2-chloroethyl)-6,7,8,9- tetrahydro-9-hydroxy-4H-pyrido[l,2-a]pyrimidin-4-one and 6-fluoro-3-(4-piperidinyl)- 1 ,2-benzisoxazole in the presence of an inorganic base. The chemical yields are slightly improved with regard to the process disclosed in EP 0 368 388, but on the other hand the organic solvents applied are very volatile and difficult to handle on industrial scale.

There is a need in the art for a new process for preparing paliperidone and its intermediates.

SUMMARY OF THE INVENTION

The present application relates to an improved process for preparing paliperidone of formula (I) by combining 3-(2-chloroethyl)-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H- pyrrido[l ,2-a]-pyrimidin-4-one (CMHTP) or a salt thereof and 6-fluoro-3-piperidino-l ,2- benzisoxazol (FBIP) or a salt thereof in an organic solvent and in the presence of water, wherein the pH value of the reaction medium is between 6 and 14, preferably between 8 and 1 1, more preferably between 8 and 10, and wherein active charcoal is used in the crystallization step(s), the active charcoal having a neutral or alkaline pH value when dispersed in water and a content of individual metal cations (e.g. Zn"⁺, Fe²⁺ cations) less than 200 ppm, leading to a product with a much reduced content of the impurities (IV and others) and wherein tedious purification steps may be omitted in comparison with prior processes for the preparation of paliperidone.

DETAILED DESCRIPTION OF THE INVENTION

The present application relates to an improved process for preparing paliperidone of formula (I) by combining 3-(2-chloroethyl)-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H- pyrrido[l,2-a]-pyrimidin-4-one (CMHTP) or a salt thereof and 6-fluoro-3-piperidino-l,2- benzisoxazol (FBIP) or a salt thereof in an organic solvent and in the presence of water, wherein the pH value of the reaction medium is between 6 and 14, preferably between 8 and 1 1, and wherein active charcoal is used in the crystallization step(s), the active charcoal having a neutral or alkaline pH value when dispersed in water, in particular the active charcoal having a neutral pH value when dispersed in water, and a content of individual metal cations (e.g. Zn²⁺, Fe²⁺ cations) less than 200 ppm, leading to a product with a much reduced content of the impurity (IV and other impurities) and wherein tedious purification steps may be omitted in comparison with prior processes for the preparation of paliperidone. Preferably, the metal cations can be Zn²⁺ and Fe²⁺. More preferably, the pH value of the reaction medium can be between 8 and 10 and the metal cations can be Zn²⁺ and Fc⁺.

Preferably, the reaction occurs in mixtures of an organic solvent, such as C 1-8 alkyl alcohols, acetonitrile, C3-6 amides, C3-6 ketones, C5-12 aromatic hydrocarbons, C2-6 alkyl acetates and C2 -8 ethers, with water. A mixture of an organic solvent and water may comprise one or two or more organic solvent(s). Preferably, a mixture of an organic solvent and water contains one organic solvent.

Preferred C 1 -4 alkyl alcohols are methanol, ethanol, n-propanol, isopropanol (IPA), n- butanol, isobutanol and 2-butanol. Preferred C3-6 amides are dimethylacetamide and dimethylformamide (DMF). Preferred C3-6 ketones are acetone, methyl ethyl ketone (MEK) and methyl iso-butyl ketone (MIBK). Preferred C2-6 alkyl acetates are ethyl acetate and isobutyl acetate. Preferred C2-8 ethers are tetrahydrofurane (THF), diethoxymethane (DEM), isobutyl methyl ether, dibutyl ether and polyethylene glycol (PGME).

Most preferably, the reaction is carried out in a mixture of methanol and water. The water may either be added to the reaction mixture directly, or indirectly, comprised in the base, e.g. diisopropylamine or diisopropylethylamine. In case it is added indirectly, the water content of the base, e.g. diisopropylamine or diisopropylethylamine, has to be determined prior to the reaction, and the missing fraction added directly to the reaction mixture.

In the process of the present invention for preparing paliperidone, the added water fraction in the solvent mixture used can be from 0.5 % (V /V) to 10% (V/V). Preferably the water fraction in the solvent mixture is from 0.5 to 5.0, most preferably from 0.5 to 2.0 % (V/V).

In the process of the present invention for preparing paliperidone, the inorganic or organic base used can be in a ratio of about 1 to about 3 moles of the base per mole of CMHTP such as about 2.5 moles of the base per mole of CMHTP. The amount of the base used, preferably, is in a molar ratio of about 2, i.e., about 2 moles of the base per mole of CMHTP, such as about 1.8 moles of the base per mole of CMHTP.

Preferably, the reaction occurs under nitrogen (oxygen is degassed from the reaction mixture) and protected from light, in order to avoid the color formation, attributed to some of the impurities. Typically, the obtained reaction mixture is heated, preferably to a temperature of about 50 °C to about reflux, most preferably to a temperature of about 65 °C to about 70 ⁰C. In a preferred embodiment, the reaction temperature is achieved by slow heating. Preferably such a temperature gradient is applied that a reaction mixture of about 200 to 300 g is heated to the reaction temperature in more than 60 minutes, preferably more than 75 minutes.

The heated mixture is preferably maintained, in particular at a temperature of about 50 ⁰C to about reflux, especially at a temperature of about 65 ⁰C to about 70 ⁰C, for at least about 5 hours, for the reaction to take place. Preferably, the reaction mixture is maintained for at least about 15 hours, and most preferably, for at least about 30 hours.

Subsequently, the reaction mixture is cooled or the product may be hot filtered. If cooling is applied, it is gradual, to about room temperature, preferably to a temperature of about 30 °C and between 30 and 90 minutes. As used herein, "room temperature" relates to a temperature of about 20 ⁰C to about 25 ⁰C.

After the cooling step, solid paliperidone is formed, which is then recovered by methods known in the art, such as filtration or centrifugation. Preferably, the obtained paliperidone is first washed with an organic solvent, which is the organic solvent used in the reaction, such as methanol, acetonitrile, acetone, dichloromethane or IPA, optionally followed by drying. Preferably, the drying is performed at about 50 ⁰C for about 3 - 6 h.

Both CMHTP and FBIP starting materials can be in the form of a base or acid addition salts. Most preferred salt is a hydrochloride salt. CMHTP may be obtained by any method known in the art, such as the ones described in EP 0 368 388, WO 2008/024415, WO 2008/140646, WO 2009/010988, WO 2009/045489 and WO 2009/047499. FBIP can generally be prepared by any known process such as the processes described in EP 0 196 132, EP 0 428 437, EP 0 081 104, J Med Chem, 1985, vol.28, no.6, p.761-769, Indian J Chem B Org, 2004, vol.43, no.9, p.1954- 1957. CMHTP may contain less than 0.5 area %, preferably less than 0.2 area % and more preferably less than 0.05 area % of impurity of the following formula (V)

The crude paliperidone according to the present invention, either still wet with a water/methanol content of 3 % w/w to below 1 % w/w directly from the filtration/centrifugation step, or first dried until the loss on drying is below 1 % w/w, is macerated in water. The ratio between the mass of the wet or dry crude paliperidone and the volume of water used is between 0.2 g/mL and 0.05 g/mL. The maceration in water is carried out at a temperature between 60 and 99 ⁰C, preferably between 85 and 95 ⁰C.

The paliperidone may further be purified, for example, by re-crystallization such as recrystallization from isopropanol, in the presence of an active charcoal having a neutral pH value when dispersed in water and a content of metal cations (each) less than 200 ppm, as determined by atomic absorption spectrometry.

The term "active charcoal having a neutral pH value" encompasses in particular active charcoals which when dispersed in an amount of 1 g of active charcoal in 50 ml of water at a temperature of 20°C (said water having before dispersion pH 7 and a temperature of 20°C) give mixtures of active charcoal and water, which mixtures preferably have a pH of more than 6 and less than 8, more preferably of more than 6.5 and less than 7.5, most preferably of more than 6.9 and less than 7.1, especially of about 7. The term "active charcoal having a neutral or alkaline pH value" encompasses in particular active charcoals which when dispersed in an amount of 1 g of active charcoal in 50 ml of water at a temperature of 2O⁰C (said water having before dispersion pH 7 and a temperature of 20°C) give mixtures of active charcoal and water, which mixtures preferably have a pH of more than 6, more preferably of more than 6.5, most preferably of more than 6.9, especially of about 7 or more, in particular a pH of more than 6 and less than 1 1 , further in particular of more than 6.5 and less than 10.

Optionally, the particle size may be optimized to the average diameter of 20 to 60 micrometers, preferably to the average diameter from 20 - 30 micrometers prior to its isolation with the use of an ultraturrax. In this way, paliperidone with a narrow distribution function is prepared, and the process is easily controlled and repeatable without the need for a second milling process. As the ultraturrax any commercially available product may be used, such as Ultra-Turrax^®. Particle size was measured using a Malvern-Mastersizer 2000 equipped with a Hydro S dispersion unit.

The term "ppm", in absence of explicit definitions to the contrary, indicates part(s) per million (weight/ weight).

The obtained crystalline form of paliperidone is characterized by powder X-ray diffraction (PXRD) pattern having peaks at about: 8.3, 10.4, 14.7, 15.1, 16.3, 18.8, 20.2, 20.8, 22.2, 24.8, 25.2 and 28.1 ± 0.2 degrees two theta.

The XRD diffractogram was recorded on a Phillips PW3040/60 X'Pert PRO diffractometer using CuKa radiation (1,541874 A).

The content of the impurity (IV) in the product obtained is below 0.15 area % as determined with the following HPLC analytical method:

Column: Gemini NX C 18, 150 x 4,6 mm, 3,0μm

Eluent A: 0.02M NaH₂PO₄, pH adjusted to 4.0 with phosphoric acid

Eluent B: mixture of acetonitrile and methanol in the ratio 30:70 Gradient:

Post time: 4 min

Flow-rate: 0.7 ml/min

Detection: UV, 238 nm

Injection volume: 5 μl

Column temperature: 40°C

Diluent: buffer pH 4.0 and acetonitrile in the ratio 1 : 1

Sample preparation:

Accurately weigh about 5 mg of sample into 10 mL volumetric flask, dissolve and dilute to volume with diluent.

Calculation:

Use area per cent method. Do not integrate solvent peaks.

In absence of any explicit indication to the contrary, a content of impurity (IV) indicated in area % or a content of impurity (V) indicated in area % can be determined using a UV detector, the detection being performed at a wavelength of 238 nm, preferably according to the above- indicated HPLC analytical method. According to another aspect, the present invention provides a paliperidone-containing mixture, which paliperidone-containing mixture comprises paliperidone of formula (I) and a compound of formula (IV). In this paliperidone-containing mixture, the content of the compound of formula (IV) can be in particular below 0.15 area %, preferably from 0.01 to below 0.15 area %, more preferably from 0.02 to 0.12 area %. Preferably, these paliperidone-containing mixtures are obtained according to any of the processes of the present invention. These paliperidone-containing mixtures, in particular when obtained according to any of the processes of the present invention, showed up to provide particular good properties, when producing medicaments on industrial scale. In the present application, the terms "the compound of formula (IV)" and "impurity (IV)" are used interchangeably.

According to yet another aspect, the present invention provides a product preferably having a content of the impurity (IV) below 0.15 area % for use in treating schizophrenia. Preferably, this product is obtained according to any of the processes of the present invention.

Furthermore, the present invention provides a paliperidone-containing mixture, which paliperidone-containing mixture comprises paliperidone of formula (I) and an impurity (IV), in which paliperidone-containing mixture the content of impurity (IV) is in particular below 0.15 area %, preferably from 0.01 to below 0.15 area %, more preferably between 0.02 and 0.12 area % for use in treating schizophrenia. Preferably, these paliperidone-containing mixtures are obtained according to any of the processes of the present invention.

According to yet another aspect, the present invention provides the use of a product preferably having a content of the impurity (IV) below 0.15 area % for the preparation of a medicament for the treatment of schizophrenia. Preferably, this product is obtained according to any of the processes of the present invention. Furthermore, the present invention provides the use of a paliperidone-containing mixture, which paliperidone-containing mixture comprises paliperidone of formula (I) and an impurity (IV), in which paliperidone-containing mixture the content of impurity (IV) is in particular below 0.15 area %, preferably from 0.01 to below 0.15 area %, more preferably between 0.02 and 0.12 area % for the preparation of a medicament for the treatment of schizophrenia. Preferably, these paliperidone-containing mixtures are obtained according to any of the processes of the present invention.

In particular, the present invention provides:

1.) A process for preparing paliperidone of formula (I) by combining 3-(2-chloroethyl)- 6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrrido[ l,2-a]-pyrimidin-4-one (CMHTP) or a salt thereof and 6-fluoro-3-piperidino-l,2-benzisoxazol (FBIP) or a salt thereof in an organic solvent and in the presence of water, wherein the pH value of the reaction medium is between 6 and 14, and wherein active charcoal is used in the crystallization step(s), having a neutral pH value and a content of individual metal cations less than 200 ppm.

2.) A process according to item 1, characterized in that the pH value of the reaction medium is between 8 and 10 and the metal cations are Zn²⁺ and Fe²⁺.

3.) A process according to items 1 or 2, characterized in that the reaction occurs in mixtures of an organic solvent, such as C 1-8 alkyl alcohols, acetonitrile, C3-6 amides, C3-6 ketones, C5-12 aromatic hydrocarbons, C2-6 alkyl acetates and C2 -8 ethers, with water.

4.) A process acoording to any of items 1 to 3, characterized in that the organic solvent is methanol. 5.) A process according to any of items 1 to 4, characterized in that the water fraction in the solvent mixture used is from 0.5 % (V/V) to 10% (V/V), preferably from 0.5 5 % (V/V) to 5.0 5 % (V/V), most preferably from 0.5 5 % (V/V) to 2.0 % 5 % (V/V).

6.) A process according to any of items 1 to 5, characterized in that the reaction mixture is heated to a temperature of about 50 °C to about reflux, preferably to a temperature of about 65 °C to about 70 °C.

7.) A process according to any of items 1 to 6, characterized in that during heating the reaction mixture such a temperature gradient is applied that a reaction mixture of about 200 to 300 g is heated to the reaction temperature in more than 60 minutes, preferably more than 75 minutes.

8.) A process according to any of items 1 to 7, characterized in that the CMHTP used contains less than 0.5%, preferably less than 0.2% and more preferably less than 0.05% of impurity of the following formula (V)

9.) A product obtained by the processes of items 1 to 8, characterized in that the content of the impurity (IV) is below 0.15 area %.

The present invention is illustrated by the following Examples without being limited thereto. Examples

Example 1

Synthesis of crude paliperidone

20.00 g of 6-fluoro-3-(4-piperidinyl)-l,2-benzisoxazol hydrochloride; 19.82 g of 3-(2- chloroethyl)-9-hydroxy-2-methyl-6,7,8,9-tetrahydro-4h-pyrido[l,2-a]pyrimidin-4-one; from 23.62 g to 27.54 g (33 mL to 38.4 mL) of dry diisopropylamine; 0.70 mL of water and 100 mL to 140 mL of methanol were charged to a 500 mL three-necked flask at room temperature. pH of suspension is 10,1. The reaction was carried out in inert atmosphere (N₂) and protected from light. The suspension was heated to 65 ⁰C to 67 ⁰C in 90 minutes until a clear solution was obtained. After one hour the reaction product began to precipitate from the reaction mixture. The reaction mixture was left at 65 - 67 °C for 30 hours, cooled to 30 ⁰C in 60 minutes, stirred for 30 minutes at 30 ⁰C and filtered. The product was washed with 40 mL of methanol. 27.5 g of paliperidone with a water content of 0.5 % were isolated. Purity was 98.6 area %, level of impurity (IV) was 0.12 area %.

Example 2

Maceration of crude paliperidone in water

10.1 g of the wet crude paliperidone obtained as described in Example 1 were suspended in 100 mL of demineralized water under inert atmosphere (N₂) and heated to 97 - 99°C. The suspension was stirred for 15 minutes and hot filtered. The product was washed with 20 mL of water and dried until LOD (Loss on Drying) was below 1% (w/w). 8.84 g of dry paliperidone were obtained. Purity was 98.8 area %, level of impurity (IV) was 0.12 area %. Example 3A

Crystallization of macerated paliperidone

9.20 g of the wet macerated paliperidone of Example 2 (containing 9.08 g of the dry product) were charged to a 1000 mL three-necked flask under inert atmosphere (N₂) and dissolved in 644 mL 2-propanol. 0.46 g of active charcoal (metal cations less than 200 ppm) was added. The mixture was heated to reflux (82 ⁰C) and stirred for 15 minutes. The suspension was hot filtered over randalite and the filtrate was cooled to room temperature. Crystallization of the product was provoked by seeding (cca 2% (w/w). The product started to precipitate between 56 °C to 50 ⁰C. After the filtration, the product was washed with 13.8 mL of 2-propanol. 8.12 g of the wet product were obtained. The product may be dried in a vacuum drier at 50 ⁰C or directly used in a second crystallization. Preferably, the wet product is directly used. Purity was 99.5 area %, level of impurity (IV) was 0.12 area %.

Example 3B

Crystallization of macerated paliperidone

7.00 g of the wet paliperidone (containing 6.93 g of the dry product) were charged to a 1000 mL three-necked flask under inert atmosphere (N₂) and dissolved in 490 mL 2- propanol. The mixture was heated to reflux (82 °C) and stirred for 15 minutes. The suspension was hot filtered over Gaf filter and the filtrate was cooled to room temperature. Crystallization of the product was provoked by seeding (cca 2% (w/w)) or with the use of the ultraturrax. The product started to precipitate between 56 ⁰C to 50 °C. After the filtration, the product was washed with 10.5 mL of 2-propanol. 8.12 g of the wet product were obtained. The product was dried in a vacuum drier at 50 ⁰C until LOD was below 1 % (w/w). 6.23 g of the dry product were obtained. Purity was 99.6 area %, level of impurity (IV) was 0.12 area %. The product obtained according to Examples 3A and 3B can be further recrystallized in order to improve the purity of the product.

Example 4

Synthesis of crude paliperidone

1.04 kg of 6-fluoro-3-(4-piperidinyl)-l,2-benzisoxazol hydrochloride, 1.03 kg of 3-(2- chloroethyl)-9-hydroxy-2-methyl-6,7,8,9-tetrahydro-4h-pyrido[l ,2-a]pyrimidin-4-one; 1.72 L of dry diisopropylamine, 0.04 L of water and 5.2 L of methanol were charged to a reactor at room temperature. The reaction was carried out in inert atmosphere (N₂) and protected from light. The suspension was heated to 65 ⁰C to 67 °C in 120 minutes until a clear solution was obtained. The reaction mixture was left at 65 - 67 °C for 30 hours, cooled to 30 °C in 125 minutes, stirred for 30 minutes at 30 ⁰C and filtered. The product was washed with 2 L of methanol. 1.69 kg of wet paliperidone (1.53 kg of dry crude paliperidone) with a solvent content of 9.6 % were isolated. Purity was 98.2 area %, level of impurity (IV) was 0.12 area %.

Example 5

Maceration of crude paliperidone in water

1.69 kg of the wet crude paliperidone obtained as described in Example 4 were suspended in 15.3 L of demineralized water under inert atmosphere (N₂) and heated to 97 - 99 °C. The suspension was stirred for 15 minutes and hot filtered. The product was washed with 1.2 L of water and dried until LOD (Loss on Drying) was below 1% (w/w). 1.4 kg of wet paliperidone were obtained. Purity is 98.5 area %, level of impurity (IV) was 0.12 area %. Example 6

Crystallization of macerated paliperidone

1.4 kg of the dry macerated paliperidone of Example 5 were charged to a reactor under inert atmosphere (N₂) and dissolved in 98 L 2-propanol. 0.06 g of active charcoal (metal cations less than 200 ppm) was added. The mixture was heated to reflux (82 °C) and stirred for 15 minutes. The suspension was hot filtered over randalite and the filtrate was cooled to room temperature. The product started to precipitate between 56 °C to 50 °C. After the filtration, the product was washed with 2 L of 2-propanol. 1.23 kg of the wet product were obtained. The product was dried in a vacuum drier at 50 ⁰C. Purity was 99.5 area %, level of impurity (IV) was 0.12 area % and mass of dry product was 1.13 kg.

Example 7

Synthesis of crude paliperidone

1.04 kg of 6-fluoro-3-(4-piperidinyl)-l,2-benzisoxazol hydrochloride, 1.03 kg of 3-(2- chloroethyl)-9-hydroxy-2-methyl-6,7,8,9-tetrahydro-4h-pyrido[l,2-a]pyrimidin-4-one; 1.72 L of dry diisopropylethylamine, 0.04 L of water and 5.2 L of methanol were charged to a reactor at room temperature. The reaction was carried out in inert atmosphere (N₂) and protected from light. The suspension was heated to 65 ⁰C to 67 °C in 140 minutes until a clear solution was obtained. The reaction mixture was left at 65 - 67 °C for 30 hours, cooled to 30 ⁰C in 50 minutes, stirred for 30 minutes at 30 °C and filtered. The product was washed with 2.5 L of methanol. 1.65 kg of wet paliperidone (1.50 kg of dry crude paliperidone) with a solvent content of 9.3 % were isolated. Purity was 98.2 area %, level of impurity (IV) was 0.06 area %. Example 8

Maceration of crude paliperidone in water

1.65 kg of the wet crude paliperidone obtained as described in Example 7 were suspended in 15 L of demineralized water under inert atmosphere (N₂) and heated to 97 - 99 °C. The suspension was stirred for 15 minutes and hot filtered. The product was washed with 1.2 L of water and dried until LOD (Loss on Drying) was below 1% (w/w). 1.5 kg of wet paliperidone were obtained. Purity is 98.6 area %, level of impurity (IV) was 0.06 area %.

Example 9

Crystallization of macerated paliperidone

1.4 kg of the dry macerated paliperidone of Example 8 were charged to a reactor under inert atmosphere (N₂) and dissolved in 98 L 2-propanol. 0.06 g of active charcoal (metal cations less than 200 ppm) was added. The mixture was heated to reflux (82 ⁰C) and stirred for 15 minutes. The suspension was hot filtered over randalite and the filtrate was cooled to room temperature. The product started to precipitate between 56 °C to 50 ⁰C. After the filtration, the product was washed with 2 L of 2-propanol. 1.1 1 kg of the wet product were obtained. The product was dried in a vacuum drier at 50 °C. Purity was 99.4 area %, level of impurity (IV) was 0.06 area % and mass of dry product was 1.1 1 kg.

Reference experiments which prove the influence of metal cations on purity of substance:

Example 10

2 g of macerated paliperidone from Example 5 were suspended in 140 mL of 2-propanol and 0.1 g of active charcoal (Zn more than 200 ppm) was added. The mixture was heated to reflux (82 °C) and stirred for 15 minutes. The suspension was hot filtered over randalite and the filtrate was cooled to room temperature. The product started to precipitate between 56 ⁰C to 50 °C. After the filtration, the product was washed with 4 mL of 2-propanol. Purity of wet product was 93.1 area %, level of impurity (IV) was 1.57 area %.

Example 11

1 g of paliperidone (level of impurity (IV) was 0.75 area %), 30 mL of water and 1.57 g ZnCI₂ (anhydrous) were charged in 50 mL three-necked flask, heated to 60 C for 30 hours and analyzed. Level of impurity (IV) was 30.2 area %

Example 12

Same as in example 1 1, but without the addition Of ZnCl₂. Level of impurity (IV) was 0.93 area %.

Example 13

Same as in example 12, but water had pH 10. Level of impurity (IV) was 0.95 area %. Example 14

Same as in example 1 1, but water had pH 10. Level of impurity (IV) was 7.5 area %. Example 15

Same as in example 12, but water had pH 3. Level of impurity (JV) was 59.9 area %. Example 16

1 g paliperidone (level of impurity (IV) was < 0, 1 area %), 10 rtiL methanol, 0,3 mL water and in so much HCl(aq) or diisopropylamin, that the pH of suspension is according to the values written in the table were charged to the flask and heated to the reflux for 24 hours. Samples of suspensions were taken and analysed. Results are presented in the table below.

Example 17

1 g of paliperidone (level of impurity (IV) was 0.75 area %), 30 mL of water and 1.86 g FeCl₂ (anhydrous) were charged in 50 mL three-necked flask, heated to 60 ⁰C for 42 hours and analyzed. Levels of impurities (RRT 0.81 and RRT 1.29) were 60 and 14 area %

Claims

CLAIMS:

1.) A process for preparing paliperidone of formula (I) by combining 3-(2-chloroethyl)- 6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrrido[ 1 ,2-a]-pyrimidin-4-one (CMHTP) or a salt thereof and 6-fluoro-3-piperidino-l ,2-benzisoxazol (FBIP) or a salt thereof in an organic solvent and in the presence of water, wherein the pH value of the reaction medium is between 6 and 14, and wherein active charcoal is used in the crystallization step(s), having a neutral or alkaline pH value and a content of individual metal cations less than 200 ppm.

2.) The process according to claim 1, characterized in that the pH value of the reaction medium is between 8 and 10 and the metal cations are Zn²⁺ and Fe²⁺.

3.) The process according to claims 1 or 2, characterized in that the reaction occurs in mixtures of an organic solvent, such as C 1-8 alkyl alcohols, acetonitrile, C3-6 amides, C3-6 ketones, C5-12 aromatic hydrocarbons, C2-6 alkyl acetates and C2 -8 ethers, with water.

4.) The process according to any of claims 1 to 3, characterized in that the organic solvent is methanol.

5.) The process according to any of claims 1 to 4, characterized in that the water fraction in the solvent mixture used is from 0.5 % (V/V) to 10% (WV), preferably from 0.5 % (V/V) to 5.0 % (V/V), most preferably from 0.5 % (V/V) to 2.0 % (V/V).

6.) The process according to any of claims 1 to 5, characterized in that the reaction mixture is heated to a temperature of about 50 ⁰C to about reflux, preferably to a temperature of about 65 °C to about 70 ⁰C.

7.) The process according to any of claims 1 to 6, characterized in that during heating the reaction mixture such a temperature gradient is applied that a reaction mixture of about 200 to 300 g is heated to the reaction temperature in more than 60 minutes, preferably more than 75 minutes.

8.) The process according to any of claims 1 to 7, characterized in that the CMHTP used contains less than 0.5%, preferably less than 0.2% and more preferably less than 0.05% of impurity of the following formula (V)

9.) The process according to any of claims 1 to 8, wherein active charcoal is used in the crystallization step(s), having a neutral pH value.

10.) A product obtained by the processes of claims 1 to 9, characterized in that the content of the impurity (IV) is below 0.15 area %.

1 1.) Paliperidone-containing mixture obtained according to the process of any of the claims 1 to 9, said paliperidone-containing mixture comprising paliperidone of formula (I) and a compound of formula (IV), the content of the compound of formula (IV) in the paliperidone-containing mixture being below 0.15 area %, preferably from 0.01 to below 0.15 area %, more preferably from 0.02 to 0.12 area %.

12.) Palperidone-containing mixture according to claim 1 1 or product according to claim 10 for use in treating schizophrenia.

13.) Use of a palperidone-containing mixture according to claim 1 1 or of a product according to claim 10 for the preparation of a medicament for the treatment of schizophrenia.